High strength steel pipe for line pipe superior in low temperature toughness and high strength steel plate for line pipe and methods of production of the same

ABSTRACT

The present invention provides high strength steel pipe for line pipe superior in low temperature toughness suppressed in drop of toughness of the HAZ and a method of production of the same, more particularly high strength steel plate for line pipe used as a material for high strength steel pipe for line pipe and a method of production of the same, in particular high strength steel pipe for line pipe superior in low temperature toughness characterized in that the chemical compositions of the base metal is, by mass %, C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mo: 0.01 to 0.15%, Al: 0.0005 to 0.030%, and Nb: 0.0001 to 0.030% contained in a range of C+0.25Si+0.1Mo+Al+Nb: 0.100% or less and the mixture of austenite and martensite present along prior austenite grain boundaries of the reheated part of the heat affected zone has a width of 10 μm or less and a length of 50 μm or less.

This application is a continuation application of U.S. application Ser.No. 12/308,677, now U.S. Pat. No. 8,764,918, filed Dec. 19, 2008, anational stage application of International Application No.PCT/JP2007/063615, filed Jul. 2, 2007, which claims priority to JapaneseApplication No. 2006-184676 filed Jul. 4, 2006, each of which isincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to steel pipe for line pipe suitable for apipeline for transporting crude oil, natural gas, etc., a steel platematerial for the same, and methods of production of the same.

BACKGROUND ART

As steel pipe for line pipe used for the trunk lines of pipelinesimportant as a long distance transport method of crude oil, natural gas,etc., high strength, high toughness steel pipe for line pipe has beenproposed (for example, Japanese Unexamined Patent Publication No.62-4826A). Up to now, commercialization of high strength steel pipe ofup to X80 in the American Petroleum Institute (API) standards has beenpushed. In recent years, however, further higher strength line pipe hasbeen demanded due to (1) the improvement in transport efficiency due tohigher pressures and (2) the improvement in on-site installationefficiency due to the reduction of the outside diameter and weight ofline pipe.

For example, if using X120 grade line pipe having a 900 MPa or highertensile strength, it is possible to increase the internal pressure, thatis, the pressure of the crude oil or natural gas, to about double thatof 65 grade line pipe, so about double the amount of crude oil ornatural gas can be transported. Further, if raising the strength of theline pipe to improve the resistance to internal pressure, compared withmaking the thickness greater, it becomes possible to cut material costs,transport costs, and on-site welding and installation costs and possibleto greatly save on pipeline laying costs.

Further, pipelines are often laid in arctic regions, so have to besuperior in low temperature toughness. Furthermore, at the time ofinstallation, the ends of the line pipes are connected, so superioron-site weldability is also demanded. To satisfy this demand, steel pipefor high strength line pipe with a base metal of a microstructure mainlycomposed of a mixed structure of bainite and martensite suitable forX120 grade line pipe higher in strength than the steel pipe for linepipe proposed in Japanese Unexamined Patent Publication No. 62-4826A hasbeen proposed (for example, Japanese Unexamined Patent Publication No.10-298707A, Japanese Unexamined Patent Publication No. 2001-303191A, andJapanese Unexamined Patent Publication No. 2004-52104A).

Furthermore, when producing steel pipe, steel plate is shaped into atube and the seam portions are seam welded. When toughness andproductivity are demanded as with steel pipe for high strength linepipe, submerged arc welding from the inner surface and outer surface ispreferable for the seam welding. When welding a steel material aplurality of times in this way, the heat affected zone (HAZ) coarsenedby the input heat from the earlier welding is reheated by the input heatof the later welding and the toughness drops.

It is known that the drop in toughness of this reheated HAZ (reheatedHAZ) is due to the formation of a mixture of martensite and austenite(MA). To solve this problem, some of the inventors proposed the methodof suppressing the drop in toughness by reducing the area ratio of theMA of the reheated HAZ and suppressing hardening of the reheated HAZ(for example, Japanese Unexamined Patent Publication No. 2004-68055A andJapanese Unexamined Patent Publication No. 2004-99930A).

However, the method proposed in Japanese Unexamined Patent PublicationNo. 2004-68055A heat treats the weld zone of the steel pipe. For thisreason, a method not requiring heat treatment of the weld zone and, inthe case of heat treatment, technology for improving the toughness at alow temperature is being demanded. Further, the method proposed inJapanese Unexamined Patent Publication No. 2004-99930A requires controlof the cooling rate after welding. Depending on the productionconditions, it is sometimes difficult to limit the cooling rate of theweld zone. For this reason, technology for improving the reheated HAZtoughness without relying on the cooling rate of the weld zone is alsobeing demanded.

DISCLOSURE OF INVENTION

The present invention provides an API standard X120 grade high strengthsteel pipe for line pipe suppressing the drop of the toughness of thereheated HAZ and superior in low temperature toughness and a method ofproduction of the same and, furthermore, high strength steel plate forline pipe able to be used as a material for high strength steel pipe forline pipe and methods of production of the same.

The inventors engaged in in-depth research focusing on the amounts of C,Si, Al, Nb, and Mo assisting the formation of MA for obtaining highstrength steel pipe for line pipe having a tensile strength in thecircumferential direction of 900 MPa or more and a superior lowtemperature toughness, in particular, low temperature toughness of theHAZ. As a result, they obtained the discovery that by controlling theamounts of C, Si, Al, Nb, and Mo to suitable ranges, the formation of MAat the prior austenite grain boundary of the reheated HAZ is suppressedand the low temperature toughness of the HAZ is improved. The presentinvention was made based on this discovery and has as its gist thefollowing:

(1) A high strength steel pipe for line pipe superior in low temperaturetoughness characterized by comprising a steel plate shaped into a tubewith seam portions of the steel plate welded by one layer each at theinner side and outer side, a base metal of the steel pipe having achemical composition containing, by mass %, C: 0.020 to 0.080%, Si: 0.01to 0.50%, Mo: 0.01 to 0.15%, Al: 0.0005 to 0.030%, and Nb: 0.0001 to0.030% in a range of C+0.25Si+0.1Mo+Al+Nb: 0.100% or less, furthercontaining, Mn: 1.50 to 2.50%, Ti: 0.003 to 0.030%, and B: 0.0001 to0.0030%, and limiting P: 0.020% or less and S: 0.0030% or less, with thebalance of Fe and unavoidable impurities, the mixture of austenite andmartensite present along prior austenite grain boundaries of thereheated parts of the heat affected zone having a width of 10 μm or lessand a length of 50 μm or less.

(2) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in (1) characterized in that the base metal has atensile strength in the circumferential direction of 900 MPa or more.

(3) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in (1) or (2) characterized in that the weldmetal has a chemical compositions containing, by mass %, C: 0.010 to0.100%, Si: 0.01 to 0.50%, Mn: 1.00 to 2.00%, Ni: 1.30 to 3.20%, Al:0.0005 to 0.100%, Ti: 0.003 to 0.050%, and O: 0.0001 to 0.0500%, furthercontaining a total of one or more of Cr, Mo, and V: 1.00 to 2.50%,limiting P: 0.020% or less and S: 0.0100% or less with the balance of Feand unavoidable impurities.

(4) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in (3) characterized in that the weld metal has achemical compositions containing, by mass %, B: 0.0001 to 0.0050%.

(5) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in any one of (1) to (4) characterized in thatthe base metal has a chemical compositions containing, by mass %, one orboth of Cu: 0.05 to 1.50% and Ni: 0.05 to 5.00%.

(6) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in any one of (1) to (5) characterized in thatthe base metal has a chemical compositions containing, by mass %, one ormore of Cr: 0.02 to 1.50%, W: 0.01 to 2.00%, V: 0.010 to 0.100%, Zr:0.0001 to 0.0500%, and Ta: 0.0001 to 0.0500%.

(7) A high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in any one of (1) to (6) characterized in thatthe base metal has a chemical compositions containing, by mass %, one ormore of Mg: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to0.0050%, Y: 0.0001 to 0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0.0001 to0.0050%.

(8) A high strength steel plate for line pipe superior in lowtemperature toughness characterized by comprising high strength steelplate for line pipe used as a material for high strength steel pipe forline pipe superior in low temperature toughness as set forth in any oneof (1) to (4) and having a chemical compositions containing, by mass %,C: 0.020 to 0.080%, Si: 0.01 to 0.50%, Mo: 0.01 to 0.15%, Al: 0.0005 to0.030%, and Nb: 0.0001 to 0.030% in a range of C+0.25Si+0.1Mo+Al+Nb:0.100% or less and further containing Mn: 1.50 to 2.50%, Ti: 0.003 to0.030%, and B: 0.0001 to 0.0030%, and limiting P: 0.020% or less and S:0.0030% or less with a balance of Fe and unavoidable impurities.

(9) A high strength steel plate for line pipe superior in lowtemperature toughness as set forth in (8) characterized by comprisinghigh strength steel plate for line pipe used as a material for highstrength steel pipe for line pipe superior in low temperature toughnessas set forth in (5) and having a chemical compositions containing, bymass %, one or both of Cu: 0.05 to 1.50% and Ni: 0.05 to 5.00%.

(10) A high strength steel plate for line pipe superior in lowtemperature toughness as set forth in (8) or (9) characterized bycomprising high strength steel plate for line pipe used as a materialfor high strength steel pipe for line pipe superior in low temperaturetoughness as set forth in (6) and having a chemical compositionscontaining, by mass %, one or more of Cr: 0.02 to 1.50%, W: 0.01 to2.00%, V: 0.010 to 0.100%, Zr: 0.0001 to 0.0500%, and Ta: 0.0001 to0.0500%.

(11) A high strength steel plate for line pipe superior in lowtemperature toughness as set forth in any one of (8) to (10)characterized by comprising high strength steel plate for line pipe usedas a material for high strength steel pipe for line pipe superior in lowtemperature toughness as set forth in (7) and having a chemicalcompositions containing, by mass %, one or more of Mg: 0.0001 to0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Y: 0.0001 to0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%.

(12) A method of production of a high strength steel plate for line pipesuperior in low temperature toughness as set forth in any one of (8) to(11), said method of production of a high strength steel plate for linepipe superior in low temperature toughness characterized by melting andcasting steel comprising the chemical compositions as set forth in anyone of (8) to (11), reheating a steel slab to 1000° C. or more, hotrolling by a reduction ratio in a nonrecrystallization temperatureregion of 3 or more, and stopping water cooling at 500° C. or lower.

(13) A method of production of high strength steel pipe for line pipesuperior in low temperature toughness characterized by comprising amethod of production of high strength steel pipe for line pipe superiorin low temperature toughness as set forth in any one of (1) to (7) andby shaping the high strength steel plate for line pipe superior in lowtemperature toughness produced by the method as set forth in any one of(8) to (11) to a tube, welding the seam portions, and then enlarging it.

(14) A method of production of high strength steel pipe for line pipesuperior in low temperature toughness as set forth in (13) characterizedby shaping the steel plate into a tube by a UO process, welding the seamportions from the inner side and outer side by submerged arc welding,then enlarging the pipe.

(15) A method of production of a high strength steel pipe for line pipesuperior in low temperature toughness as set forth in any one of (12) to(14) characterized in that a welding wire used for the submerged arcwelding as set forth in (14) has a chemical compositions containing, bymass %, C: 0.01 to 0.12%, Si: 0.05 to 0.50%, Mn: 1.00 to 2.50%, and Ni:2.00 to 8.50%, further containing one or more of Cr, Mo, and V in arange of Cr+Mo+V: 1.00 to 5.00% with a balance of Fe and unavoidableimpurities.

(16) A method of production of high strength steel pipe for line pipesuperior in low temperature toughness as set forth in (15) characterizedin that the chemical compositions of the welding wire is, by mass %, B:0.0001 to 0.0050%.

(17) A method of production of high strength steel pipe for line pipesuperior in low temperature toughness as set forth in any one of (12) to(16) characterized by heat treating the weld zone and heat affected zoneafter welding and before pipe expansion.

(18) A method of production of high strength steel pipe for line pipesuperior in low temperature toughness as set forth in any one of (13) to(17) characterized in that the heating temperature when heat treatingthe weld zone and heat affected zone is 200 to 500° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a reheated HAZ.

FIG. 2 is a view showing the effects of the chemical compositions on thetoughness of the reheated HAZ.

FIG. 3 is a schematic view of a reheated HAZ of a weld metal.

FIG. 4 is a schematic view of martensite or bainite.

FIG. 5 is a schematic view of granular bainite.

BEST MODE FOR CARRYING OUT THE INVENTION

First, the low temperature toughness of the HAZ will be explained. Asshown schematically in FIG. 1, the reheated HAZ 1 is the location wherethe weld metal and HAZ near the weld line of the earlier welding arereheated by later welding. While differing somewhat by the input heat atthe time of welding, normally the HAZ is the location within 10 mm fromthe weld line. At the reheated HAZ, there is sometimes coarse MA 2 alongprior austenite grain boundaries 3. If this becomes the starting pointof fracture, the low temperature toughness remarkably falls. For thisreason, it is difficult to improve the toughness of the HAZ at the partof one-half of the thickness of the steel pipe for high strength linepipe. In particular, when providing a notch at a location correspondingto the reheated HAZ, for example, a position 1 mm or 2 mm from the weldline, the Charpy absorption energy at −40° C. sometimes becomes lessthan 50 J.

The inventors engaged in intensive research to clarify the relationshipbetween the effects of elements assisting the formation of MA on the lowtemperature toughness of the heat affected zone, that is, the amounts ofaddition of C, Si, Al, Nb, and Mo, and the toughness of the HAZ. First,they took samples from steel materials comprised of various chemicalcompositions and performed heat treatment simulating the heat history ofthe reheated HAZ (reheated HAZ reproduction test). This involves heatingthe steel material to 1400° C. and immediately cooling it to roomtemperature and further heating it to 750° C. and immediately cooling itto room temperature during which making the cooling rate from 750° C. to500° C. at the time of cooling 5 to 10° C./s. The inventors took V-notchtest pieces from the steel materials after the reheated HAZ reproductiontest based on JIS Z 2242 and ran Charpy impact tests at −40° C. and −60°C. The results of the toughness evaluated by the reheated HAZreproduction test are shown in FIG. 2.

FIG. 2 shows the relationship between the amount of C+0.25Si+0.1Mo+Al+Nband the Charpy absorption energy at −40° C. and −60° C. of the reheatedHAZ obtained by the simulation tests. From FIG. 2, it becomes clear thatif possible to suppress the amount of C+0.25Si+0.1Mo+Al+Nb to 0.100% orless, the Charpy absorption energy of the reheated HAZ at −40° C. and−60° C. becomes 50 J or more.

Next, the inventors investigated the effects of the amounts of additionof C, Si, Al, Nb, and Mo on the formation of MA at the reheated HAZ. Inthe same way as evaluation of the low temperature toughness, they tooksamples from steel materials subjected to the reheated HAZ reproductiontest, mechanically polished and Nital etched them, then observed them bya scanning electron microscope (SEM). In this observation by an SEM, theMA present along prior austenite grain boundaries were white overall, socould be discerned. The inventors measured the size of the MA formedalong prior austenite grain boundaries and as a result learned thatunder conditions where the toughness evaluated by the reheated HAZreproduction test is good, the MA has a width of 10 μm or less and alength of 50 μm or less.

Based on the above discoveries, the inventors engaged in further studiesand as a result confirmed that if suppressing the amount of C to 0.080%or less, preferably 0.060% or less, Si to 0.50% or less, Mo to 0.15% orless, and Al and Nb to 0.030% or less and making the total of theC+0.25Si+0.1Mo+Al+Nb 0.100% or less, the coarsening of the MA formedalong the coarsened prior austenite grain boundaries in the reheated HAZis suppressed and the Charpy absorption energy at −40° C. and −60° C. isimproved to 50 J or more.

The inventors studied the toughness of the reheated weld metal in thesame way as the reheated HAZ. The reheated weld metal, as shownschematically in FIG. 3, is the location where the weld metal at thecenter part in the circumferential direction formed by earlier weldingis reheated by later welding. The reheated weld metal 4, while differingsomewhat by the input heat at the time of welding, is usually thelocation within 5 mm distance from the weld line of the later welding atthe center part in the circumferential direction formed by earlierwelding.

In the reheated weld metal as well, in the same way as the reheated HAZ,there is coarse MA present along prior austenite grain boundaries andthis becomes the starting point of fracture and remarkably lowers theCharpy absorption energy in some cases. Regarding the reheated weldmetal as well, if suppressing the amount of C to 0.100% or less, Si to0.50% or less, preferably 0.40% or less, Al to 0.100% or less, andCr+Mo+V to 2.50% or less, coarsening of MA formed along prior austenitegrain boundaries is suppressed. If taking a test piece at a locationincluding the reheated weld metal, for example, centered at the meetingpart of the earlier welding and later welding and providing notches atthe center part of the circumferential direction of the weld metal, forexample, the Charpy absorption energy at −40° C. and −60° C., becomes 50J or more.

Below, the reasons for limitation of the chemical compositions of thematrix material of the high strength steel pipe and the steel plate ofthe material of the steel pipe of the present invention will beexplained.

C is a basic element improving the strength of the steel and has to beadded in 0.020% or more. On the other hand, by excessive addition of Cover 0.080%, the steel material falls in weldability and coarse MA isformed at the reheated HAZ causing a drop in toughness, so the upperlimit of the amount of C was made 0.080% or less. From the viewpoint ofthe low temperature toughness and the strength, the preferable range ofthe amount of C is 0.030 to 0.060%.

Si is a deoxidizing element and has to be added in an amount of 0.01% ormore. On the other hand, if the amount of Si exceeds 0.50%, coarse MA isformed at the reheated HAZ causing a drop in toughness, so the upperlimit was made 0.50% or less.

Mo is an element improving the hardenability and forming carbonitridesto improve the strength. To obtain this effect, addition of 0.01% ormore is necessary. On the other hand, if adding a large amount of Moexceeding 0.15%, the strength rises and the toughness falls. Further,coarse MA is formed at the reheated HAZ and the toughness is impaired,so the upper limit is made 0.15% or less.

Al is a deoxidizing element and has to be added in an amount of 0.0005%or more. To sufficiently reduce the amount of oxygen, it is preferableto add Al in an amount of 0.001% or more. On the other hand, if addingAl in over 0.030%, coarse MA is formed at the reheated HAZ and thetoughness falls, so the upper limit is made 0.030% or less.

Nb is an element forming carbides and nitrides and effective forimproving the strength. To obtain this effect, addition of 0.0001% ormore is necessary. To sufficiently improve the strength, addition of0.001% or more of Nb is preferable. On the other hand, if adding Nb inover 0.030%, coarse MA is formed at the reheated HAZ and the toughnessfalls, so the upper limit is made 0.030% or less.

Furthermore, in the present invention, it is necessary thatC+0.25Si+0.1Mo+Al+Nb be 0.100% or less. This is because if theC+0.25Si+0.1Mo+Al+Nb exceeds 0.100%, coarse MA is formed at the reheatedHAZ and the toughness falls. The lower limit of C+0.25Si+0.1Mo+Al+Nb is0.0241% since the lower limits of C, Si, Mo, Al, and Nb are respectively0.020%, 0.01%, 0.01%, 0.0005%, and 0.0001%. Further, the preferablelower limits of Al and Nb are both 0.001%, so the preferable lower limitof C+0.25Si+0.1Mo+Al+Nb is 0.0255%.

Mn is an element required for adjusting the strength and toughness ofthe steel. If less than 1.50%, securing the strength becomes difficult,while if over 2.50%, the toughness of the HAZ falls. For this reason,the amount of addition of Mn is made 1.50 to 2.50%.

Ti is a deoxidizing element. Further, it is an element forming nitridesand exhibiting an effect on the refinement of the crystal grains. Toobtain this effect, addition of 0.003% or more is necessary. On theother hand, addition of over 0.030% causes a remarkable drop intoughness due to the formation of carbides, so the upper limit is made0.030%.

B is an element increasing the hardenability when in solid solution andlowering the solid solution N and thereby improving the toughness of theHAZ when precipitating as BN. To improve the balance of strength andtoughness, the amount of addition has to be made 0.0001 to 0.0030%.

P is an impurity. If contained in an amount of over 0.020%, the matrixmaterial of the steel pipe is remarkably impaired in toughness, so theupper limit is made 0.020% or less. To suppress the drop in toughness ofthe HAZ of steel pipe, the upper limit of the P content is preferablymade 0.010% or less.

S is also an impurity. If contained in an amount over 0.0030%, coarsesulfides are produced and the toughness is impaired, so the upper limitwas made 0.0030%.

Note that in the present invention, as elements for improving thestrength and toughness, one or more elements of Cu, Ni, Cr, V, Zr, andTa may be added.

Cu is an element effective for improving the strength without causing adrop in the toughness, but if the content is less than 0.05%, asufficient effect is not obtained in some cases, while if over 1.50%,cracks easily occur at the time of heating the steel slab or the time ofwelding. Therefore, the content of Cu is preferably made 0.05 to 1.50%.

Ni is an element effective for improvement of the toughness andstrength. To obtain this effect, it is preferable to add 0.05% or more.On the other hand, if adding Ni in over 5.00%, the weldability isimpaired in some cases, so the upper limit is preferably made 5.00% orless.

Cr is an element contributing to the improvement of the strength of thesteel by precipitation strengthening. Addition of 0.02% or more ispreferable. On the other hand, if adding Cr in an amount over 1.50%, thehardenability is raised, a bainite structure is formed, and thetoughness is impaired in some cases, so the upper limit is preferablymade 1.50%.

W is an element improving the hardenability and improving the strength.To obtain these effects, addition of 0.01% or more is preferable. On theother hand, if adding a large amount of W over 2.0%, the strength risesand the toughness falls. Further, to suppress the formation of coarse MAat the reheated HAZ, the upper limit is preferably made 0.50% or less.

V, Zr, and Ta are elements forming carbides and nitrides andcontributing to the improvement of the strength. The lower limits arepreferably made 0.010% or more, 0.0001% or more, and 0.0001% or more.The preferable lower limits of the Zr and Ta for sufficiently obtainingthe effect of improvement of the strength are both 0.001% or more. Onthe other hand, if V, Zr, and Ta are excessively added, the toughness issometimes impaired, so the upper limits of the V, Zr, and Ta arepreferably made respectively 0.100% or less, 0.0500% or less, and0.0500% or less.

Furthermore, to control the forms of the oxides and inclusions, one ormore of Mg, Ca, REM, Y, Hf, and Re may be added.

Mg is effective as a deoxidizing element. Addition of 0.0001% or more ispreferred. Further, Mg acts as in-grain transformation and pinningparticles and contributes to the refinement of the grains of the steeland HAZ, so to obtain this effect, addition of 0.0010% or more ispreferable. On the other hand, if adding Mg over 0.0100%, coarse oxidesare easily formed and the toughness of the base metal and HAZ isimpaired in some cases, so the upper limit is preferably made 0.0100% orless.

Ca, REM, Y, Hf, and Re are elements forming sulfides and are effectivein particular for suppressing the formation of MnS stretched in therolling direction. To obtain the effect of improvement of thecharacteristics in the thickness direction of the steel material, inparticular, the lamellar tear resistance, the lower limits of the amountof addition of Ca, REM, Y, Hf, and Re are preferably made 0.0005% ormore. On the other hand, if the amounts of addition of Ca, REM, Y, Hf,and Re exceed 0.0050%, they form coarse inclusions and impair thetoughness in some cases, so the upper limits are preferably made 0.0050%or less.

Steel containing the above chemical compositions is produced by thesteelmaking process, then made into a slab by the continuous castingprocess and made into steel plate by hot rolling. In the presentinvention, the hot rolling is important. The steel slab is reheated,then rolled at the recrystallization temperature or higher for“recrystallization rolling”, then is rolled at less than therecrystallization temperature and in the austenite range for“nonrecrystallization rolling”. The hot rolling has to be performedunder the following conditions so as to make the structure of the steelplate finer, preferably to make prior austenite average particle size 20μm or less.

When hot rolling the steel slab, the temperature of the reheating ismade 1000° C. or more. This is because if performing the hot rolling ata temperature where the structure of the steel becomes a singleaustenite phase, that is, in the austenite region, the crystal grainsize of the steel plate is made finer. The upper limit is not defined,but to suppress coarsening of the prior austenite grains, the reheatingtemperature is preferably made 1250° C. or less.

The reduction ratio of the nonrecrystallization rolling is made 3 ormore. Due to this, the prior austenite becomes finer in crystal grainsize and the average particle size becomes 20 μm or less. Note that inthe present invention, the reduction ratio of the nonrecrystallizationrolling means the ratio of the thickness before the start ofnonrecrystallization rolling divided by the thickness afternonrecrystallization rolling.

Further, the reduction ratio of the recrystallization rolling ispreferably made 2 or more to refine the crystal grain size of the prioraustenite. Note that in the present invention, the reduction ratio ofthe recrystallization rolling means the ratio of the thickness of thesteel slab divided by the thickness after the recrystallization rolling.Further, no upper limit of the reduction ratio of thenonrecrystallization rolling and recrystallization rolling is defined,but if considering the thickness of the steel slab before rolling andthe thickness of the steel slab after rolling, it is usually 10 or less.

Furthermore, after the end of the rolling, the steel plate is watercooled. If making the temperature of stopping the water cooling 500° C.or lower, a superior strength and toughness can be obtained. No lowerlimit of the temperature for stopping the water cooling is defined. Thewater cooling may be performed even down to room temperature, but ifconsidering the productivity and hydrogen defects, 150° C. or more ispreferable.

The metal structure of the thus obtained steel plate has an area ratioof bainite or an area ratio of bainite and martensite of 80% or more anda balance of a total of one or more of granular bainite, polygonalferrite, and MA of 20% or less. The steel pipe produced using this steelplate as a material has a tensile strength in the circumferentialdirection of 900 MPa or more, a superior low temperature toughness aswell, and a Charpy absorption energy measured at −40° C. of 200 J ormore.

When shaping the steel plate into a tube, then arc welding the seamportions to obtain a steel pipe, the shaping is preferably by the UOEprocess of C-pressing, U-pressing, and O-pressing the steel plate. TheUOE process is a production process suitable for shaping steel pipe forline pipe of a high strength and a thickness of 10 to 30 mm.

For the arc welding, submerged arc welding is preferably employed fromthe viewpoint of the toughness of the weld metal and productivity. Ifusing steel plate made of the chemical compositions of the presentinvention as a material, even if performing submerged arc welding, withits large input heat of welding, from the inner surface and outersurface of the steel pipe, it is possible to make the width of the MAformed along the prior austenite grain boundaries of the reheated HAZ 10μm or less and make the length 50 μm or less. Further, when performingsubmerged arc welding, the input heat is preferably made 10.0 kJ/mm orless. Due to this, the prior austenite average particle size of the HAZbecomes 500 μm or less and the MA formed along the prior austenite grainboundaries of the reheated HAZ can be further reduced in width andlength.

The MA can be observed by taking a sample from the reheated HAZ,mechanically polishing and Nital etching it, and viewing it by an SEM.The MA should be observed by an SEM by a power of 1000× to 10000×. Thesmaller the width and length of the MA present along prior austenitegrain boundaries, the more preferable. The lower limit is not defined,but if less than 0.1 μm, discrimination becomes difficult.

Further, the wire used for the welding is preferably made the followingelements to make the chemical compositions of the weld metal the laterexplained ranges considering dilution of the elements by the base metal.That is, it may contain, by mass %, C: 0.01 to 0.12%, Si: 0.05 to 0.5%,Mn: 1.0 to 2.5%, and Ni: 2.0 to 8.5%, further contain one or more of Cr,Mo, and V in a range of Cr+Mo+V: 1.0 to 5.0%, and have a balance of Feand unavoidable impurities. B: 0.0001 to 0.0050% may also be included.

Furthermore, the chemical compositions of the weld metal will beexplained.

C is an element extremely effective for improving the strength. 0.010%or more is preferably included. However, if the amount of C is toogreat, weld cold cracking easily occurs. In particular, the HAZsometimes hardens and the toughness is impaired at the so-called T-crossparts where the on-site weld zone and seam welding cross. For thisreason, the upper limit of the amount of C is preferably made 0.100%. Toimprove the toughness of the weld metal, the upper limit is morepreferably made 0.050% or less.

Si prevents the formation of the welding defects of blowholes, so ispreferably included in an amount of 0.01% or more. On the other hand, ifexcessively included, the low temperature toughness is remarkablydegraded, so the upper limit is preferably made 0.50% or less. Inparticular, when welding a plurality of times, the low temperaturetoughness of the reheated weld metal is sometimes degraded, so the upperlimit is more preferably made 0.40% or less.

Mn is an element effective for securing a superior balance of strengthand toughness. The lower limit is preferably made 1.00% or more.However, if Mn is included in a large amount, segregation is promotedand the low temperature toughness is degraded. Not only that, productionof welding wire used for welding becomes difficult. Therefore, the upperlimit is preferably made 2.00% or less.

Ni is an element raising the hardenability to secure strength andfurther improving the low temperature toughness. It is preferablyincluded in an amount of 1.30% or more. On the other hand, if thecontent of Ni is too great, high temperature cracking sometimes occurs,so the upper limit was made 3.20% or less.

Al is an element added for improving the refining and solidificationwhen producing welding wire. It is also added to the base metal, so0.0005% or more is sometimes included. Further, to actively utilize thefine Ti-based oxides and suppress coarsening of the grains of the weldmetal, 0.001% or more of Al is preferably contained. However, Al is anelement promoting the formation of MA, so the preferable upper limit ofthe content is 0.100% or less.

Ti is an element forming micro oxides and refining the grains of theweld metal and is preferably included in an amount of 0.003% or more. Onthe other hand, if Ti is included in a large amount, a large amount ofcarbides of Ti are produced and the low temperature toughness isdegraded, so the upper limit is preferably made 0.050% or less.

O is an impurity. The amount of oxygen finally remaining in the weldmetal is usually 0.0001% or more. However, when the amount of 0 remainsin over 0.0500%, the coarse oxides become more numerous and thetoughness of the weld metal sometimes drops, so the upper limit ispreferably made 0.0500% or less.

Cr, Mo, and V are all elements raising the hardenability. For highstrength of the weld metal, among these, one or more of these arepreferably included in a total of 1.00% or more. On the other hand, ifthe total of the one or more of Cr, Mo, and V exceeds 2.50%, the lowtemperature toughness sometimes deteriorates, so the upper limit ispreferably made 2.50% or less.

P and S are impurities. To reduce the deterioration of the lowtemperature toughness of the weld metal and cold crack susceptibility,the respective upper limits are preferably made 0.020% and 0.0100% orless. Note that from the viewpoint of the low temperature toughness, themore preferably upper limit of P is 0.010%.

The weld metal may further contain B.

B is an element increasing the hardenability of the weld metal. To raisethe strength, it is preferably contained in an amount of 0.0001% ormore. On the other hand, if the content of the B exceeds 0.0050%, thetoughness is sometimes impaired, so the upper limit is preferably made0.0050% or less.

If making the chemical compositions of the weld metal the above range,it is possible to make the MA formed along the prior austenite grainboundaries of the reheated weld metal a width of 10 μm or less and alength of 50 μm or less. Furthermore, to refine the MA, it is preferableto perform submerged arc welding with an input heat of 10.0 kJ/mm orless.

When running tensile tests in the circumferential direction at thelocations of the steel pipe including the weld metal as well, thetensile strength is preferably 900 MPa or more. For this reason,preferably the strength of the weld metal is made higher than that ofthe base metal, softening of the HAZ is suppressed, and location ofbreakage in the tensile test is made the base metal. To make thestrength of the weld metal higher than that of the base metal andimprove the toughness of the weld metal, it is preferable to make themetal structure of the weld metal one with an area ratio of bainite andan area ratio of bainite and martensite of 80% or more and with thebalance of the total of one or more of granular bainite, polygonalferrite, and MA of 20% or less.

When using an optical microscope to observe the structures of the steelplate and base metal and weld metal of the steel pipe, the cross-sectionin the circumferential direction of the steel pipe or width direction ofthe steel plate is made the observed cross-section, mechanicallypolished, then etched by Nital. The sample used for observation by theoptical microscope is preferably prepared and the average particle sizeof the prior austenite measured by the sectioning method of JIS G 0551.The metal structure of bainite and martensite seen in the case ofobserving the metal structures of the steel plate and base metal andweld metal of the steel pipe of the present invention by an opticalmicroscope is shown schematically in FIG. 4.

FIG. 4(a) shows a metal structure also called “lower bainite” which iscomprised of fine laths 5 and fine cementite 6 precipitated in thelaths. Note that in observation of the structure by an opticalmicroscope, martensite, in the same way as FIG. 4(a), is also comprisedof fine laths 5 and fine cementite 6 precipitated in the laths. FIG.4(b) shows a metal structure also called “pseudo upper bainite”. It haslaths of greater widths than the lower bainite of the FIG. 4(a).Further, it does not have fine cementite in the laths, but has MAbetween the laths 5. In the present invention, “bainite” is the generalterm for lower bainite of the form schematically shown in FIG. 4(a) andthe pseudo upper bainite of the form schematically shown in FIG. 4(b).

Note that when using an optical microscope to observe a metal structure,both martensite and lower bainite have the forms schematically shown inFIG. 4(a), so discrimination is difficult. Note that martensite andbainite and ferrite and granular bainite can be discriminated by anoptical microscope. Granular bainite resembles acicular ferrite. Asschematically shown in FIG. 5, it has coarser MA than pseudo upperbainite and, unlike bainite, has granular ferrite 7 present.

Further, to make the tensile strength in the circumferential directionof the steel pipe 900 MPa or more and secure a good toughness, it ispreferable to make one or both of carbon equivalent Ceq calculated fromthe chemical compositions of the base metal and weld metal and thehardenability indicator Pcm suitable ranges. The carbon equivalent Ceqis calculated by the following (formula 1). In the base metal, it is inthe range of 0.20 to 0.80, while in the weld metal, it is preferably0.60 to 1.30. If considering the balance of the strength and toughness,in the base metal, it is more preferably 0.30 to 0.70 in range and, inthe weld metal, 0.70 to 1.20:Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (formula 1)

where, C, Mn, Cu, Ni, Cr, Mo, and V are contents (mass %) of theelements.

Further, the hardenability indicator Pcm is calculated by the following(formula 2). In the base metal, it is preferably 0.100 to 0.300 inrange, while in the weld metal, it is 0.200 to 0.500. If considering thebalance of the strength and toughness, in the base metal, it is morepreferably 0.150 to 0.250 in range and, in the weld metal, 0.250 to0.400:Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5×B  (formula 2)

where, C, Si, Mn, Cu, Cr, Ni, Mo, V, and B are contents (mass %) of theelements.

Note that when the contents of the optionally contained elements Cu, Ni,Cr, and V are less than the preferable lower limits, in (formula 1) and(formula 2), the carbon equivalent Ceq and hardenability indicator Pcmare calculated assuming these to be 0.

The weld zone and HAZ of the steel pipe are preferably further heattreated. If heating to a temperature of 200 to 500° C., the coarse MAformed along the prior austenite grain boundaries breaks up into bainiteand fine cementite and the toughness is improved. With a heatingtemperature of less than 200° C., the breakup of the coarse MA isinsufficient and the effect of improvement of the toughness is notsufficient sometimes, so the lower limit is preferably made 200° C. ormore. On the other hand, if heating the weld zone to over 500° C.,precipitate is formed and the toughness of the weld metal deterioratesin some cases, so the upper limit is preferably made 500° C. or less. Ifthe MA formed at the reheated HAZ breaks up into bainite and cementite,while the shapes are similar to MA in observation by SEM, the insidescontain fine white precipitate making differentiation from MA possible.

The heat treatment of the weld zone and HAZ may be performed by heatingfrom the outer surface by a burner or by high frequency heating. Afterthe outermost surface reaches the heat treatment temperature, the pipemay be immediately cooled, but to promote the breakup of MA, the pipe ispreferably held there for 1 sec to 300 sec. However, if considering thecost of the facilities and the productivity, the holding time ispreferably 180 sec or less.

EXAMPLES

Below, the present invention will be explained in detail using examples.

Example 1

Steels made of the chemical compositions of Table 1 and Table 2(continuation of Table 1) were produced and continuously cast to obtainsteel slabs having thicknesses of 240 mm. The blank spaces in Table 1mean the content of the ingredient is less than the detectable limit.These steel slabs were heated to 1100 to 1210° C., hot rolled at 950° C.or higher recrystallization temperatures to a thickness of 100 mm, thenstarted to be rolled at 880° C. by nonrecrystallization rolling. Therolling was ended at 750° C. to obtain thicknesses of 13 to 25 mm. Watercooling was started in the temperature range of 670 to 750° C. The watercooling was stopped at 300° C.

The obtained steel plates were shaped into tubes by the UOE process,then the seam portions were welded from the inner surface and outersurface by one layer each by submerged arc welding. The chemicalcompositions of the welding wires were ones containing, by mass %, C:0.01 to 0.12%, Si: 0.05 to 0.5%, Mn: 1.0 to 2.5%, and Ni: 2.0 to 8.5%,further containing one or more of Cr, Mo, and V to the range of Cr+Mo+V:1.0 to 5.0%, and having a balance of Fe and unavoidable impurities. Partof the welding wires further contain B: 0.0001 to 0.0050%. The inputheat of the welding was made 2.0 to 5.0 kJ/mm.

Using the position 1 mm from the boundary between the weld metal and theHAZ, that is, the weld line, as the observation position, the metalstructure of the HAZ was observed based on JIS G 0551 and the averageparticle size of the old austenite was measured by the sectioningmethod. Further, a Nital etched sample was observed by an SEM andmeasured for width and length of the MA. The tensile strength in thecircumferential direction of the base metal was measured and a Charpyimpact test of the HAZ conducted based on the API Standard 5L. TheCharpy impact test of the HAZ was conducted so that the position of thenotch became the reheated HAZ. Specifically, a V-notch was introduced toa location of 1 mm from the bonded part and the test conducted at −40°C. and −60° C. The results are shown in Table 3. Note that the tensilestrength of the circumferential direction measured using the weld metalat the center part of the test piece is equivalent to the tensilestrength of the matrix material. It was confirmed that the location ofbreakage was the base metal.

Further, samples were taken from the weld metal of some of the steelpipes and analyzed for chemical compositions. The results are shown inTable 4. The blank spaces in Table 4 show that the content of theingredient is less than the detectable limit. Furthermore, using theposition of 1 mm from the meeting part of the earlier welding and laterwelding at the center part in the circumferential direction of the weldmetal of these steel pipes as the observed position, the structure ofthe weld metal formed by the earlier welding, that is, the metalstructure of the reheated weld metal, was observed based on JIS G 0551and the prior austenite average particle size was measured by thesectioning method. Furthermore, a Nital etched sample was observed by anSEM and the width and length of the MA measured. The tensile test andCharpy impact test of the weld metal were conducted based on APIStandard 5L. The Charpy impact test of the weld metal was performed sothat the position of the notch became the reheated weld metal.Specifically, test pieces were tested at −40° C. and −60° C. whileintroducing a V-notch at the center at the meeting part of the earlierwelding and later welding of the weld metal. The results are shown inTable 5.

Furthermore, for some of the samples, samples containing weld metal andHAZ were taken, measured for the temperature of the surface by athermocouple, and heat treated by a burner from the outer surface of theweld zone and HAZ. Note that samples were not held at the heat treatmenttemperature. They were immediately cooled after reaching the heattreatment temperature. The samples were heat treated under theconditions shown in Table 6, then measured for width and length of theMA at the reheated HAZ and subjected to Charpy impact tests of the HAZ.Further, the tensile strength in the circumferential direction wasmeasured using the weld metal as the center part of the test piece. Theresults are shown in Table 5. The weld metal was heat treated under theconditions shown in Table 7, then measured for width and length of theMA at the reheated weld metal and subjected to a Charpy impact test andtensile test. The results are shown in Table 7.

In Table 3, the Steel Pipe Matrix Material Nos. B1 to B22 show examplesof the present invention. These steel plates all have high tensilestrengths, fine MA observed at the prior austenite grain boundaries ofthe reheated HAZ, superior low temperature toughness of the HAZ, andCharpy absorption energy of the HAZ at −40° C. and −60° C., shown byvE⁻⁴⁰ and vE⁻⁶⁰, of over 50 J.

On the other hand, the Steel Pipe Matrix Material Nos. B23 to B31 arecomparative examples having chemical compositions outside the range ofthe present invention. All have C+0.25Si+0.1Mo+Al+Nb over 0.100%, so theMA of the reheated HAZ coarsens and the toughness of the HAZ falls.Further, the Steel Pipe Matrix Material No. B23 has an amount of C lowerthan the range of the present invention, so the tensile strength falls.The Steel Pipe Matrix Material Nos. B26 and B27 have contents of P and Sover the ranges of the present invention, so the toughness of the HAZremarkably falls.

The Weld Metal Nos. W1 to W14 shown in Table 5 are invention exampleswith ingredients of the weld metal in the preferable ranges. For thisreason, the MA of the reheated weld metal is fine and the Charpyabsorption energy of the weld metal at −40° C. and −60° C. shown byvE⁻⁴⁰ and vE⁻⁶⁰ is over 50 J. On the other hand, the Weld Metal Nos. W15to W20 are comparative examples with chemical compositions of the basemetal outside the ranges of the present invention and with chemicalcompositions of the weld metal outside the preferable ranges. Further,the Weld Metal Nos. W21 to W25 are reference examples with chemicalcompositions of the weld metal outside the preferable ranges.

The Weld Metal No. W15 has an amount of C smaller than the preferablerange, so the tensile strength falls. The Weld Metal Nos. W16 and W17respectively have an amount of C and an amount of Mn over the preferableranges, so the strength rises, the MA of the reheated weld metalcoarsens, and the toughness of the weld metal falls. The Weld Metal No.W18 has an amount of P and No. W19 an amount of S in excess of thepreferable ranges, so are examples of a drop in toughness of the weldmetal. The Weld Metal No. W20 has an amount of Ti in excess of thepreferable range, so Ti carbides are formed and the toughness of theweld metal falls.

The Weld Metal No. W21 has an amount of Si and No. W22 an amount of Alover the preferable ranges, so the MA of the reheated weld metalcoarsens and the toughness of the weld metal falls. The Weld Metal No.W23 has an amount of Ni greater than the preferable range. While good instrength and toughness, high temperature cracks occur. The Weld MetalNo. W24 has an amount of Cr+Mo+V smaller than the preferable range, sothe tensile strength falls, while the Weld Metal No. W25 has an amountof Cr+Mo+V over the preferable range, so the strength rises, the MA ofthe reheated weld metal coarsens, and the toughness of the weld metalfalls.

In Table 6, the Steel Pipe Matrix Material Nos. B1 to B19 have heattreatment temperatures in the preferable range. Compared with beforeheat treatment, the tensile strength in the circumferential directionfalls, the MA of the reheated HAZ breaks up and becomes finer, and thetoughness is improved. On the other hand, the Steel Pipe Matrix MaterialNo. B20 has a heat treatment temperature lower than the preferablerange, so the effects of refinement of the MA and improvement of thetoughness are not remarkable. Further, the Steel Pipe Matrix MaterialNos. B21 and B22 have heat treatment temperatures higher than thepreferable range. While some breakup of MA is recognized, the toughnessfalls compared with before heat treatment.

The Weld Metal Nos. W1 to W11 shown in Table 7 have heat treatmenttemperatures within the preferable range. Compared with before heattreatment, the tensile strength falls, the MA of the reheated weld metalbreaks up and becomes finer, and the toughness rises. On the other hand,the Weld Metal No. W12 has a heat treatment temperature lower than thepreferable range, so the effects of refinement of the MA and improvementof the toughness are not remarkable. Further, the Weld Metal Nos. W13and W14 have heat treatment temperatures higher than the preferabletemperature. While some breakup of MA is observed, the toughness doesnot fall compared with before heat treatment.

TABLE 1 Steel Plate Chemical Compositions (mass %) No. C Si Mn P S Mo NbAl Ti B Cu Ni A 0.035 0.10 1.95 0.005 0.0005 0.09 0.025 0.004 0.0120.0010 0.50 B 0.040 0.10 1.81 0.008 0.0006 0.04 0.016 0.013 0.003 0.00050.40 C 0.040 0.08 1.90 0.003 0.0008 0.07 0.017 0.008 0.012 0.0030 0.300.30 D 0.046 0.07 2.12 0.004 0.0003 0.06 0.016 0.010 0.016 0.0003 0.300.80 E 0.044 0.11 1.85 0.009 0.0006 0.01 0.014 0.007 0.012 0.0020 0.30 F0.045 0.10 1.85 0.026 0.0004 0.01 0.013 0.015 0.012 0.0004 0.35 0.35 G0.036 0.02 1.80 0.003 0.0006 0.15 0.030 0.005 0.013 0.0015 H 0.035 0.011.90 0.007 0.0005 0.08 0.030 0.013 0.008 0.0003 0.30 0.50 I 0.036 0.101.90 0.005 0.0002 0.07 0.015 0.013 0.010 0.0008 0.20 J 0.045 0.11 2.200.008 0.0004 0.11 0.012 0.004 0.030 0.0008 0.30 0.30 K 0.046 0.11 1.850.002 0.0003 0.01 0.020 0.004 0.024 0.0026 L 0.048 0.11 2.12 0.0040.0006 0.02 0.014 0.010 0.012 0.0006 0.40 M 0.035 0.11 1.86 0.006 0.00080.05 0.015 0.015 0.024 0.0025 N 0.046 0.12 2.12 0.006 0.0006 0.05 0.0150.001 0.013 0.0005 0.30 0.35 O 0.040 0.08 2.00 0.004 0.0004 0.07 0.0150.017 0.012 0.0010 0.30 0.50 P 0.035 0.18 2.00 0.003 0.0006 0.01 0.0130.006 0.008 0.0003 0.40 0.80 Q 0.040 0.08 1.96 0.002 0.0006 0.06 0.0180.003 0.010 0.0019 R 0.040 0.13 2.20 0.004 0.0006 0.08 0.016 0.003 0.0050.0003 0.30 0.50 S 0.052 0.06 2.30 0.007 0.0003 0.01 0.013 0.016 0.0260.0030 0.40 T 0.038 0.07 2.20 0.003 0.0005 0.10 0.014 0.020 0.012 0.00031.00 0.80 U 0.036 0.12 1.80 0.002 0.0008 0.08 0.016 0.003 0.017 0.00260.05 V 0.038 0.10 1.96 0.004 0.0025 0.01 0.015 0.020 0.018 0.0002 1.00 W0.005 0.18 2.20 0.005 0.0026 0.40 0.030 0.005 0.012 0.0026 0.50 0.80 X0.210 0.45 1.75 0.007 0.0015 0.01 0.030 0.016 0.013 0.0003 Y 0.040 0.123.50 0.015 0.0021 0.02 0.030 0.017 0.008 0.0029 Z 0.060 0.25 1.93 0.0400.0026 0.60 0.030 0.009 0.019 0.0002 AA 0.045 0.17 1.86 0.003 0.03510.30 0.021 0.005 0.017 0.0027 0.30 0.30 AB 0.060 0.05 1.96 0.005 0.00300.30 0.030 0.100 0.023 0.0060 AC 0.059 0.09 2.40 0.003 0.0009 0.30 0.0300.003 0.064 0.0002 0.60 AD 0.046 0.12 1.85 0.019 0.0008 0.02 0.030 0.0140.002 0.0021 0.40 0.13 AE 0.060 1.50 1.96 0.002 0.0015 0.30 0.030 0.0030.010 0.0003 0.50 1.50

TABLE 2 (Continuation of Table 1) Steel Chemical Compositions (mass %)Plate Cr, W, V, Mg, Ca, REM, C + 0.25Si + Re- No. Zr, Ta Y, Hf, Re Al +0.1Mo + Nb Ceq Pcm marks A Cr: 0.30, V: 0.060 Mg: 0.0053 0.098 0.4830.176 Inv. B V: 0.060 Ca: 0.0012 0.098 0.388 0.152 ex. C V: 0.040 REM:0.0008 0.092 0.419 0.181 D V: 0.050, Zr: 0.0051 0.096 0.495 0.193 E Cr:0.30, V: 0.060, 0.094 0.446 0.177 Ta: 0.0032 F Cr: 0.60, Zr: 0.0012 Ca:0.0021 0.099 0.522 0.197 G W: 0.50, V: 0.060 Mg: 0.0038 0.091 0.3780.150 H Cr: 0.30, V: 0.100 Ca: 0.0022 0.089 0.501 0.186 I V: 0.050 0.0960.390 0.151 J Cr: 0.60, V: 0.070 Mg: 0.0018 0.100 0.608 0.227 Ca: 0.0024K 0.099 0.356 0.156 L Cr: 0.50, V: 0.060, 0.102 0.544 0.200 Zr: 0.0137 MCr: 0.40, V: 0.090 0.098 0.453 0.177 N Cr: 0.60 Mg: 0.0033 0.097 0.5730.213 Ca: 0.0035 O Y: 0.0010 0.099 0.441 0.176 P Cr: 0.70 REM: 0.00070.100 0.590 0.212 Q Cr: 0.30, Zr: 0.0008 Re: 0.0025 0.087 0.439 0.169 RTa: 0.0229 0.099 0.476 0.185 S Cr: 0.30 REM: 0.0006 0.097 0.524 0.206 TCr: 0.60, V: 0.050 Mg: 0.0025 0.100 0.675 0.257 Ca: 0.0017 U V: 0.050Hf: 0.0020 0.093 0.365 0.156 V Cr: 1.00 Ca: 0.0021 0.099 0.633 0.208 WCr: 0.50, V: 0.050 0.125 0.648 0.229 Comp. X V: 0.200 Ca: 0.0013 0.3700.544 0.335 ex. Y REM: 0.0012 0.119 0.627 0.235 Z Cr: 0.60, V: 0.0600.221 0.634 0.242 AA Cr: 0.30 Mg: 0.0005 0.144 0.515 0.212 AB Cr: 0.30,V: 0.080 0.233 0.523 0.233 AC Cr: 0.30 Ca: 0.0017 0.145 0.619 0.228 AD0.122 0.394 0.177 AE Cr: 0.30 REM: 0.0007 0.498 0.640 0.295

TABLE 3 Steel Base Pipe MA of reheated metal Toughness of base SteelThick- HAZ Tensile HAZ metal Plate ness Width Length strength vE⁻⁴⁰vE⁻⁶⁰ Re- No. No. mm μm μm MPa J J marks B1 A 14 4 30 915 100 80 Inv. B2B 25 3 20 923 90 70 ex. B3 C 16 2 15 945 94 74 B4 D 15 4 24 932 125 65B5 E 19 6 26 915 110 74 B6 F 17 5 32 910 95 75 B7 G 15 4 24 950 105 75B8 H 16 5 15 920 91 71 B9 I 21 4 16 940 112 82 B10 J 23 2 18 925 110 70B11 K 15 9 24 956 87 67 B12 L 16 7 22 1000 110 70 B13 M 17 5 26 950 11777 B14 N 19 6 24 945 101 71 B15 O 24 4 31 923 94 74 B16 P 16 2 34 924 9070 B17 Q 19 8 24 940 120 60 B18 R 20 4 25 915 87 67 B19 S 16 3 27 926 8969 B20 T 21 1 29 930 91 71 B21 U 13 2 32 915 110 70 B22 V 17 4 14 915130 74 B23 W 15 11  30 750 25 15 Comp. B24 X 16 15  70 1150 13 5 ex. B25Y 21 12  80 1020 46 26 B26 Z 23 11  30 915 5 5 B27 AA 15 16  20 925 5 5B28 AB 17 11  55 915 20 0 B29 AC 19 14  45 930 48 28 B30 AD 23 13  40915 26 10 B31 AE 16 17  60 900 25 5

TABLE 4 Weld Steel Chemical Compositions (mass %) Metal plate Cr + Mo +Re- No. code C Si Mn P S Ti Al Ni Cr Mo V B O V Ceq Pcm mark W1 A 0.0550.25 1.63 0.005 0.0005 0.020 0.010 3.00 1.00 1.36 0.035 0.0003 0.02452.40 1.006 0.341 Inv. W2 B 0.044 0.20 1.43 0.008 0.0006 0.018 0.008 2.901.00 1.50 0.060 0.0005 0.0285 2.41 0.988 0.329 ex. W3 C 0.100 0.08 1.400.003 0.0008 0.025 0.005 2.50 1.00 1.20 0.040 0.0003 0.0213 2.24 0.9480.350 W4 D 0.070 0.05 1.50 0.004 0.0003 0.026 0.004 2.40 0.88 1.23 0.0500.0006 0.0265 2.16 0.912 0.321 W5 E 0.060 0.35 1.52 0.009 0.0006 0.0210.056 3.00 0.90 1.32 0.060 0.0010 0.0323 2.28 0.969 0.342 W6 F 0.0700.10 1.25 0.005 0.0004 0.026 0.003 2.45 1.11 1.11 0.040 0.0006 0.02452.26 0.894 0.313 W7 G 0.045 0.02 1.80 0.003 0.0006 0.021 0.015 2.65 1.000.95 0.060 0.0015 0.0198 2.01 0.924 0.307 W8 H 0.080 0.15 1.80 0.0070.0005 0.026 0.001 1.40 0.95 1.50 0.030 0.0006 0.0178 2.48 0.969 0.352W9 I 0.070 0.17 1.65 0.005 0.0002 0.030 0.003 3.00 0.85 1.45 0.0500.0008 0.0426 2.35 1.015 0.356 W10 J 0.060 0.30 1.95 0.008 0.0004 0.0350.012 3.00 0.60 0.40 0.070 0.0351 2.35 0.799 0.281 W11 K 0.040 0.22 1.400.002 0.0003 0.024 0.001 2.80 1.30 1.50 0.060 0.0026 0.0215 2.41 1.0320.348 W12 L 0.050 0.25 1.56 0.004 0.0006 0.004 0.005 2.50 1.30 1.230.060 0.0198 2.32 0.995 0.331 W13 M 0.060 0.31 1.43 0.006 0.0008 0.0450.012 2.50 1.50 1.24 0.050 0.0025 0.0234 2.41 1.023 0.359 W14 N 0.0600.20 1.56 0.006 0.0006 0.025 0.007 2.13 1.23 1.12 0.000 0.0045 0.01952.35 0.932 0.339 W15 W 0.005 0.28 1.42 0.004 0.0004 0.026 0.013 2.561.02 1.12 0.000 0.0010 0.0234 2.14 0.840 0.259 Co. W16 X 0.200 0.32 1.450.003 0.0006 0.023 0.002 2.45 1.13 1.12 0.000 0.0195 2.25 1.055 0.455ex. W17 Y 0.050 0.24 2.50 0.004 0.0006 0.020 0.006 2.56 1.03 1.05 0.0500.0003 0.0189 2.13 1.063 0.354 W18 Z 0.080 0.31 1.60 0.025 0.0008 0.0200.026 2.56 1.12 1.02 0.050 0.0026 0.0256 2.19 0.955 0.355 W19 AA 0.0800.31 1.63 0.004 0.0200 0.025 0.021 2.85 1.13 1.12 0.040 0.0006 0.02342.29 1.000 0.358 W20 AC 0.060 0.15 1.56 0.002 0.0006 0.100 0.032 2.851.03 1.03 0.040 0.0002 0.0256 2.10 0.930 0.316 W21 O 0.100 0.80 1.450.003 0.0005 0.035 0.024 3.00 0.96 0.96 0.050 0.0243 1.97 0.936 0.366Ref. W22 P 0.070 0.18 1.70 0.005 0.0021 0.016 0.150 0.50 1.05 0.96 0.0500.0026 0.0267 2.06 0.799 0.304 ex. W23 Q 0.060 0.15 1.75 0.007 0.00020.018 0.021 4.00 0.89 0.86 0.030 0.0005 0.0321 1.78 0.974 0.327 W24 R0.040 0.12 1.65 0.005 0.0003 0.020 0.003 2.56 0.0029 0.0248 0.486 0.184W25 S 0.060 0.25 1.63 0.005 0.0006 0.036 0.033 2.85 3.50 0.60 0.0600.0216 4.16 1.354 0.418

TABLE 5 Mechanical properties MA of reheated of weld metal Weld Steelweld metal Tensile Metal plate Width Length strength vE⁻⁴⁰ vE⁻⁶⁰ Re- No.Code μm μm MPa J J mark W1 A 2 20 1056 120 95 Inv. W2 B 3 10 1037 130105 ex. W3 C 4 15 995 125 100 W4 D 2 10 958 135 110 W5 E 4 30 1018 145120 W6 F 6 20 1012 124 99 W7 G 2 14 1031 135 110 W8 H 5 15 1018 105 80W9 I 4 12 1066 124 99 W10 J 3 15 939 125 100 W11 K 2 13 1084 150 125 W12L 5 24 1044 110 85 W13 M 4 25 1074 151 126 W14 N 6 14 979 115 90 W15 W 225 882 130 105 Co. W16 X 11 55 1108 30 5 ex. W17 Y 12 53 1117 30 5 W18 Z6 15 1003 24 10 W19 AA 4 12 1050 45 20 W20 AC 9 46 977 45 20 W21 O 13 55982 35 10 Ref. W22 P 15 65 902 25 0 ex. W23 Q 4 15 1023 110 90 W24 R 415 870 42 17 W25 S 11 51 1221 10 5

TABLE 6 Heat Circum- treat- ferential Steel ment direc- Pipe tem- MA ofreheated tion Toughness of base Steel per- HAZ Tensile HAZ metal Plateature Width Length strength vE⁻⁴⁰ vE⁻⁶⁰ Re- No. No. ° C. μm μm MPa J Jmark B1 A 350 3 25 905 130 90 Inv. B2 B 400 2 15 913 120 80 ex. B3 C 4501 10 935 124 84 B4 D 500 3 19 922 155 75 B5 E 300 5 21 905 140 84 B6 F350 4 27 900 125 85 B7 G 400 3 19 940 135 85 B8 H 450 4 10 910 121 81 B9I 440 3 11 930 142 92 B10 J 320 1 13 915 140 80 B11 K 350 8 19 946 11777 B12 L 280 6 17 990 140 80 B13 M 350 4 21 940 147 87 B14 N 420 5 19935 131 81 B15 O 400 3 26 913 124 84 B16 P 380 1 29 914 120 80 B17 Q 4807 19 930 150 70 B18 R 280 3 20 905 117 77 B19 S 420 2 22 916 119 79 B20T 180 1 28 938 92 72 B21 U 570 1 27 905 55 40 B22 V 620 3 9 905 60 45

TABLE 7 Heat treat- Mechanical properties ment MA of reheated of weldmetal Weld Steel temper- weld metal Tensile Metal plate ature WidthLength strength vE⁻⁴⁰ vE⁻⁶⁰ Re- No. Code ° C. μm μm MPa J J mark W1 A350 1 15 1046 165 120 Inv. W2 B 400 2 5 1027 175 130 ex. W3 C 450 3 10985 170 125 W4 D 500 1 5 948 180 135 W5 E 300 3 25 1008 190 145 W6 F 3505 15 1002 169 124 W7 G 400 1 9 1021 180 135 W8 H 450 4 10 1008 150 105W9 I 440 3 7 1056 169 124 W10 J 320 2 10 929 170 125 W11 K 350 1 8 1074195 150 W12 L 180 5 23 1046 112 87 W13 M 570 3 20 1064 75 45 W14 N 620 59 980 55 40

INDUSTRIAL APPLICABILITY

According to the present invention, it becomes possible to provide anAPI standard X120 grade high strength steel pipe for line pipesuppressing the toughness of the HAZ of steel pipe and a method ofproduction of the same and furthermore a high strength steel plate forline pipe able to be used as a material for high strength steel pipe forline pipe and a method of production of the same.

The invention claimed is:
 1. A high strength steel plate for line pipehaving a chemical composition containing, by mass %, C: 0.020 to 0.080%,Si: 0.01 to 0.50%, Mo: 0.01 to 0.09%, Al: 0.0005 to 0.030%, and Nb:0.013 to 0.030% in a range of C+0.25Si+0.1Mo+Al+Nb: 0.100% or less andfurther containing Mn: 1.50 to 2.50%, Ti: 0.003 to 0.030%, and B: 0.0001to 0.0030%, and limiting P: 0.020% or less, S: 0.0030% or less with abalance of Fe and unavoidable impurities.
 2. A high strength steel platefor line pipe as set forth in claim 1, wherein the chemical compositionfurther contains, by mass %, one or both of Cu: 0.05 to 1.50% and Ni:0.05 to 5.00%.
 3. A high strength steel plate for line pipe as set forthin claim 1, wherein the chemical composition further contains, by mass%, one or more of Cr: 0.02 to 1.50%, W: 0.01 to 2.00%, V: 0.010 to0.100%, Zr: 0.0001 to 0.0500%, and Ta: 0.0001 to 0.0500%.
 4. A highstrength steel plate for line pipe as set forth in claim 1, wherein thechemical composition further contains, by mass %, one or more of Mg:0.0001 to 0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Y:0.0001 to 0.0050%, HE 0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%.
 5. Ahigh strength steel plate for line pipe as set forth in claim 2, whereinthe chemical composition further contains, by mass %, one or more of Cr:0.02 to 1.50%, W: 0.01 to 2.00%, V: 0.010 to 0.100%, Zr: 0.0001 to0.0500%, and Ta: 0.0001 to 0.0500%.
 6. A high strength steel plate forline pipe as set forth in claim 2, wherein the chemical compositionfurther contains, by mass %, one or more of Mg: 0.0001 to 0.0100%, Ca:0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Y: 0.0001 to 0.0050%, Hf:0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%.
 7. A high strength steelplate for line pipe as set forth in claim 3, wherein the chemicalcomposition further contains, by mass %, one or more of Mg: 0.0001 to0.0100%, Ca: 0.0001 to 0.0050%, REM: 0.0001 to 0.0050%, Y: 0.0001 to0.0050%, Hf: 0.0001 to 0.0050%, and Re: 0.0001 to 0.0050%.
 8. A methodof production of a high strength steel plate for line pipe as set forthin claim 1, said method of production of a high strength steel plate forline pipe characterized by melting and casting steel comprising thechemical composition as set forth claim 1, reheating a steel slab to1000° C. or more, hot rolling by a reduction ratio in anonrecrystallization temperature region of 3 or more, and stopping watercooling at 500° C. or lower.
 9. A method of production of a highstrength steel plate for line pipe as set forth in claim 2, said methodof production of a high strength steel plate for line pipe characterizedby melting and casting steel comprising the chemical composition as setforth in claim 2, reheating a steel slab to 1000° C. or more, hotrolling by a reduction ratio in a nonrecrystallization temperatureregion of 3 or more, and stopping water cooling at 500° C. or lower. 10.A method of production of a high strength steel plate for line pipe asset forth in claim 3, said method of production of a high strength steelplate for line pipe characterized by melting and casting steelcomprising the chemical composition as set forth in claim 3, reheating asteel slab to 1000° C. or more, hot rolling by a reduction ratio in anonrecrystallization temperature region of 3 or more, and stopping watercooling at 500° C. or lower.
 11. A method of production of a highstrength steel plate for line pipe as set forth in claim 4, said methodof production of a high strength steel plate for line pipe characterizedby melting and casting steel comprising the chemical composition as setforth in claim 4, reheating a steel slab to 1000° C. or more, hotrolling by a reduction ratio in a nonrecrystallization temperatureregion of 3 or more, and stopping water cooling at 500° C. or lower.